Well, from what I've been reading on the Apple forums, I'm not alone. Seems some iMacs have this exact issue and at this point I think we can rule out temperature.
I too have worked with electronics, including computers for many, many years in some pretty cold places (read that below zero for days on end) and have never seen a temperature related failure that wasn't related to bad solder joints or something else involving expansion/contraction of a connection point.
I have seen machines refuse to run properly if too cold, but this condition corrects itself in short order. Condensation can be a problem, but only if there's a significant amount of moisture in the air. I judge the potential for condensation by looking at the equipment when it's brought in from the cold. If there is evidence of condensation on the case or enclosure I'll wait to fire it up. No condensation, no problem.
I'm pretty confident that in this case we have a funky iMac.